The locomotive and a rake of vacuum-fitted stock will appear in place of the WHHR's flagship loco, Russell, which is currently undergoing restoration.
"When we heard that Russell's return to service had been delayed, we felt the obligation to help our neighbours from the other end of Porthmadog, says F&WHR General Manager Paul Lewin. "We would encourage all our supporters and members to pay a visit over the May Day weekend to help the WHHR's fundraising efforts.
"We look forward to seeing Russell in steam on its return later in the year."
It will be the first time that a Garratt has travelled on WHHR metals and provides a unique opportunity to see Garratts in steam on two adjacent railways. K1 and its rake of carriages is being made available to the WHHR on a no-cost basis.